[PATCH][cxxabi] ARM EHABI zero-cost exception handling for libc++abi

[Nick Kledzik]

On Apr 13, 2014, at 3:16 AM, Logan Chien <tzuhsiang.chien at gmail.com> wrote:

> Hi,
> 
> I have worked on ARM EHABI zero-cost exception handling for libc++abi.
> 
> With the attached patch, I can compile and run all libc++abi unittests on ARM Linux.
> 
> Is it OK to commit?  Thanks.
> 

> diff --git a/include/unwind.h b/include/unwind.h
> index c5acd93..e57d756 100644
> --- a/include/unwind.h
> +++ b/include/unwind.h
> @@ -23,8 +23,24 @@
>  #define LIBUNWIND_UNAVAIL
>  #endif
>  
> +#if !defined(LIBCXXABI_SJLJ)
> +#  if defined(__arm__)
> +#    if defined(__APPLE__)
> +#      define LIBCXXABI_ARM_EHABI 0
> +#      define LIBCXXABI_SJLJ 1
> +#    else
> +#      define LIBCXXABI_ARM_EHABI 1
> +#      define LIBCXXABI_SJLJ 0
> +#    endif
> +#  else
> +#    define LIBCXXABI_ARM_EHABI 0
> +#    define LIBCXXABI_SJLJ 0
> +#  endif
> +#endif
I recently found that the compiler has a built-in define  __USING_SJLJ_EXCEPTIONS__ that is set when compiling for setjump/longjump based exceptions.  Is there are similar built-in for ARM EHABI?  If so, we can set these up without testing for __arm__ or __APPLE__.  In fact, LIBCXXABI_SJLJ can be replaced with __USING_SJLJ_EXCEPTIONS__.  And perhaps LIBCXXABI_ARM_EHABI can be replaced with some built-in?

> diff --git a/src/cxa_exception.cpp b/src/cxa_exception.cpp
> index 744dddd..27a55c5 100644
> --- a/src/cxa_exception.cpp
> +++ b/src/cxa_exception.cpp
> @@ -232,7 +232,7 @@ __cxa_throw(void* thrown_object, std::type_info* tinfo, void (*dest)(void*))
>      globals->uncaughtExceptions += 1;   // Not atomically, since globals are thread-local
>  
>      exception_header->unwindHeader.exception_cleanup = exception_cleanup_func;
> -#if __arm__
> +#if LIBCXXABI_SJLJ
>      _Unwind_SjLj_RaiseException(&exception_header->unwindHeader);
>  #else
I like the conditional test for SJLJ instead of the processor, but as I recently discovered, most of the existing __arm__ tests can be changed to __USING_SJLJ_EXCEPTIONS__ tests.

-Nick